A major purpose of protective eyewear and glasses is to protect the eyes from dust and dirt and wind drying. When air flows around the lenses and past the eyes it carries with it dust, dirt, pollens, bacteria and particulates. In motorcycle goggles fine dirt and dust can filter through the airflow channels to the eye cavities and stick to the eyes of the user. In ski goggles fine particle snow can penetrate the filters. In safety goggles fine particulates can be entrained in the airflow through the vents and reach the eyes and surface of interior lens, irritating the eyes and obscuring vision. In all these eye-wear systems the main purpose is to block direct projectile impacts from colliding with the eyes of the user by using protective lenses while the goggles still must be ventilated to remove moisture and prevent fogging of the lenses.
The problem of fogging in sports goggles has been known for many years. In ski goggles this effect takes place when the interior of the lens reaches the dew point from the combination of cooling of the lens in contact with the cold outside air, and the flux of moisture into the interior air volume from the face. The fundamental concept to reduce fogging in goggles is to achieve a counter action to the effects of the heat removal through the lens and have the temperature of the air in the goggle be above the dew point. Attempts at making non-fogging goggles are quite numerous and typically rely on a variety of different techniques, but the most common is to have an open cell foam air filter along the perimeter gasket of the goggle. The cells of this foam are open enough to let air pass through while blocking the snow crystals. If the cells of the foam are very small or the foam is thick, it has a high resistance to airflow and can lead to insufficient airflow and fogging will occur in the goggles. If the foam is thin or the cells large in diameter the airflow is sufficient to prevent fogging but the finer ice crystals can pass through.